1. Technical Field
The present invention relates to a passive intermodulation (PIM) measurement instrument with circuitry configured to reduce average DC power needed during operation.
2. Related Art
A PIM is an unwanted signal or signals generated by the non-linear mixing of two or more frequencies in a passive device such as a connector or cable. PIM has surfaced as a problem for cellular telephone technologies such as Global System for Mobile Communications (GSM), Advanced Wireless Service (AWS) and Personal Communication Service (PCS) systems. Cable assemblies connecting a base station to an antenna on a tower using these cellular systems typically have multiple connectors that cause PIMs that can interfere with system operation.
The PIM signals are created when two signals from the same or different systems mix at a PIM point such as a faulty cable connector. If the generated PIM harmonic frequency components fall within the receive band of a base station, it can effectively block a channel and make the base station receiver think that a carrier is present when one is not. PIMs can, thus, occur when two base stations operating at different frequencies, such as an AWS device and a PCS device, are in close proximity.
The PIMs can be reduced or eliminated by replacing faulty cables or connectors. Test systems are thus utilized to detect the PIMs enabling a technician to locate the faulty cable or connector. The test system to measure the PIMs, thus, creates signals at two different frequencies, amplifies them, and provides them through cables connecting a base station to antennas on a tower for the base stations. A return signal carrying the PIMs is filtered to select a desired test frequency harmonic where PIMs can be detected and identified to an operator.
PIM testers to date have used continuous wave (CW) signals for the two frequencies used to create the PIM. This is due to the unknown nature of where physically the PIM is located in the transmission path. The PIM is monitored by one technician while the other technician climbs the tower and physically moves the connector joints to see if the PIM changes. Other techniques plot a time graph of the PIM so a single technician can correlate his movement up the tower with results on a graph provided on a plotter below the tower.
FIG. 1 shows a block diagram of components of a prior art test system setup for measuring a PIM. The test system utilizes two signal sources 2 and 12 producing continuous wave (CW) signals, with a first signal source 2 producing a signal at frequency F1 and the second signal source 12 producing a signal at frequency F2. When these multiple signals are allowed to share the same signal path in a nonlinear transmission medium, the unwanted signals can occur. The combined 3rd order response is particularly troublesome as it produces an unwanted signal at 2F1-F2 that can pass from one system transmitter into another system's receiver.
The signal at frequency F1 is provided from source 2 to a high power amplifier (HPA) 4. The signal at frequency F2 is provided from source 12 to a high power amplifier 14. Both the high power amplifiers 4 and 14 are shown as 50 W amplifiers, and receive a DC power supply input shown ranging from 100 to 125 Watts to produce a 50 Watt signal output.
The output of each of the amplifiers 4 and 14 is provided through respective isolators 6 and 16 to the input of a power combiner 20. The combiner 20 assures the two carrier signals F1 and F2 are isolated from each other. If they are allowed to combine without isolation, intermodulations would appear due to power output stage nonlinearities. The isolators 6 and 16 are inserted after the power amplifiers 4 and 14 to give additional isolation from any return signal from the combiner 20. The intermodulations are the same frequency as the PIM (2F1-F2), so isolation using both the combiner 20 and the isolators 6 and 16 is critical.
The outputs of the power combiner 20 are provided to duplexer 22. The duplexer 26 provides the signals F1 and F2 to one terminal, while the signal 2F1-F2 is provided to another terminal. The signal 2F1-F2 can be provided to a digital receiver or spectrum analyzer (not shown) for measurement.
The power needed to create the PIM is a standardized 20 W per carrier. Overall for the PIM test circuit of FIG. 1, the DC power supplied to the amplifiers 4 and 14 needs to be 500 to 625% higher to create the two 20 W output due to efficiency of amplifiers 4 and 14, and RF losses through the isolators 6 and 16, combiner 20 and duplexer 22. This translates to a continuous DC power consumption of 200 to 250 Watts. The loss of power combiner 20 is 3 dB, so 25 W carriers (F1 and F2) can emerge from the combiner while other 25 W carriers that are not needed are dissipated in an internal load 21. The 50 Watt power output of the two amplifiers 4 and 14 is further reduced a total of at least 1 dB above the theoretical 3 dB loss through the combiner 20 due to the losses through cabling. Further losses in the isolators 6 and 16 and duplexer 22 reduce total power so that 20 Watt carriers F1 and F2 are produced from the output of duplexer 22 and provided through cable 17 to antenna 18. PIMs introduced by the cable 17, antenna 18 or other sources receiving the signals F1 and F2 will generate a return signal 2F1-F2 that is provided back through duplexer 22 and directed to a PIM test receiver for processing.
FIG. 2 shows an alternative prior art circuit that reduces the total power needed to provide the 20 Watt PIM carrier signal. The circuit of FIG. 2 accomplishes the power savings by eliminating the hybrid combiner 20 and instead using a frequency duplexer 30. The frequency duplexer 30 combines the two carriers F1 and F2 while isolating them from each other to create outputs at the fixed frequencies F1 and F2 with losses of 0.3 dB rather than the 3 to 4 dB needed by the power combiner/splitter 20 for a savings of 100 to 125 Watts. Components carried over from FIG. 1 to FIG. 2 are similarly labeled, as will be components carried over in subsequent figures.
In FIG. 2, without the 3 to 4 dB loss of a power combiner instead of a 50 Watt output from amplifiers 4 and 14, they can have a 25 Watt output and still produce 20 Watt PIM carrier signals. The DC power supply used by the amplifiers 4 and 14 is, thus, reduced by half from 100-125 Watts in FIG. 1, to 50-62.5 Watts in FIG. 2. It would be desirable to provide even more power savings for a PIM measurement circuit.